docs: def write_strand(self, strand):
          """ Writes a Strand object to the stream. """
          if strand['channel'] != self._current_channel \
             or self._strand_counter == self._reads_per_file:
              self._start_new_file(strand)
          fname = self._write_strand(strand)
          self._index.write('{}\t{}\t{}\t{}\n'.format(strand['channel'],
                                                      strand['read_attrs']['read_number'],
                                                      self._current_file, fname))
          return